Fired bullets and bullet cartridges are left with markings from the firearm from which they come. These markings result from forced contact with metal parts in the firearm and are unique to the firearm since the metal parts have engraved surfaces which are themselves unique due to the machining process during manufacturing. Using microscopic examination of fired bullets and cartridges (casings or shells), firearms experts have traditionally been able to compare markings to identify whether fired bullets or spent cartridges originate from a given firearm.
The process of bullet examination has been successfully automated using apparatus as set out in PCT/CA92/00216 (published as WO 92/20988). Bullet illumination is described in U.S. Pat. No. 5,379,355.
In manual comparison of fired cartridges, the cartridge under investigation is placed under a microscope along with a test cartridge from a known firearm. The impression of the breech face on the primer and the firing pin impression in the primer surface have markings that are compared. Conventionally, side illumination is used which provides an image dependent on the direction of the illumination. To compare the markings on cartridge bases, the angle of illumination must be the same. For this purpose, illumination direction protocols are used. When two cartridges originate from the same firearm, the markings will have the same characteristics. Since the appearance of the primer breech face impression and firing pin impression is different using different angles of illumination, the firearms examiner will usually confirm a match using illumination from more than one angle.
The need to use an illumination direction protocol and possibly additional images of each cartridge using different illumination angles is a problem for automating the process of imaging and analysis.